The manufacture of most modem electronic products requires a printed circuit board (PCB) that allows to electrically interconnect a variety of electronic components and also holds them together in a relatively rigid condition. Many types of components are placed over a single PCB. Electronic components such as resistors, capacitors, inductors, transformers, integrated circuit (IC) packages, connectors, headers, RF shields, LEDs, switches, board interface systems, battery sockets, etc. are electrically connected and restrained by means of soldered joints. In general, these joints are obtained by three methods: hand soldering, the wave soldering process and by the reflow soldering process.
Manufacturing electronic products around PCBs requires a few sequential steps performed by different machines. For example, such steps may comprise: (1) printing the PCB with soldering paste (an operation generally performed by stencil printing equipment), (2) placing surface-mount electronic components on that PCB face (an operation performed by an automated computer-controlled "pick-and-place" machine or by any other type of component placement equipment), (3) soldering the assembly (an operation, until now, performed inside a reflow oven or by a wave soldering machine), (4) cleaning the completed assembly (an operation that may involve washing and drying) and (5) testing the assembly for proper functionality (detects components damaged during step (3) and the presence of defective soldered joints.) Rework or rejection may be required after operation (5).
Mass production exclusively utilizes wave and reflow soldering processes, either individually or in combination to accomplish above step (3). Both processes exhibit inherit disadvantages that indeed, increase the cost of the final product, generate rejects, require rework and reduce the reliability of the final product. The electronic manufacturing industry accepts these inherit drawbacks and shortcomings, and works around them, for lack of a more suitable soldering process.
Both wave and reflow soldering processes simultaneously heat up the entire electronic product, meaning the PCB and all of the components being soldered to the PCB, to a temperature ranging from about 20.degree. C. (degree Celsius) to 40.degree. C. above the temperature at which the utilized solder alloy melts or reaches liquidus state. The melting temperature of solder alloys utilized by the electronic industry ranges from 190.degree. C. to 300.degree. C.
The majority of consumer electronic products need to be rated, and indeed are, to operate at maximum temperatures that range from 50.degree. C. to 90.degree. C. Consequently, components that form part of every electronic product manufactured by either wave or reflow soldering processes are required to survive temperatures from 120.degree. C. to 290.degree. C. higher than those temperatures encountered during their most severe actual operation. Therefore, all electronic components must be unnecessarily temperature-overrated to tolerate or survive the harsh soldering process. This requirement for high-temperature-exposure survival increases the cost of every component to be soldered to a PCB.
During the soldering process, thermal shock (due to a fast heating rate) can crack certain components, in particular ceramic capacitors, increasing rejects and/or requiring costly rework. Fast heating of plastic IC packages could induce cracking when moisture absorbed inside said packages can turn into steam during a reflow soldering process causing the so called "pop-corn" effect that internally damage the IC package. Electrolytic capacitors are extremely sensitive to high temperature exposure. Laminated PCBs may become soft by extended exposure to heat. An increase in soldering process temperature can damage a PCB metal-plated through-holes or vias, by cracking their barrels due to differential thermal expansion between the PCB dielectric material and its barrels' plating metal. Also, the difference in coefficient of thermal expansion between board and ceramic capacitors can result in significant stress at the soldered joints which may induce cracking. Warpage, or twisting of a PCB, increases with soldering temperature. Warpage can cause defective soldered joints because coplanarity of the mating surfaces is compromised. In addition, defective joints result due to movement of the components from their intended soldering pads location. During a reflow soldering operation, components can move due to the following effects: liquefied-solder surface tension, liquid-solder induced buoyancy, convective gas flow, equipment generated noise and vibration and other well known factors.
Recently, electrically conductive adhesives are becoming increasingly prominent in electronics packaging applications in large part because their ability to provide electrical interconnection without the need to subject the component to the harsh high-temperature environment of a reflow soldering process. Heat sensitive components that could be damaged during reflow process are being electrically interconnected by conductive adhesives. This type of electrical interconnection is not as desirable as traditional soldered joints.
In conclusion, the cost of manufacturing electronic products around PCBs can be reduced and the quality and reliability of said products improved by using the new soldering apparatus and process disclosed in my copending parent patent application Ser. No. 09/396,923. That new soldering process only heats the soldering pads (or lands) on a PCB and the mating leads (or terminations) extending out from electronic component casings, while allowing said casings to remain relatively cold. That novel soldering process permits the elimination of all the disadvantages enumerated above.
When this inventor realized the need to create an alternative device to efficiently implement the process disclosed by said parent application, the objectives and purposes of this invention were inspired, leading him to the conception and the accomplishment of this invention.